1. Field of the Invention
The present invention relates to a friction plate for a wet type multiplate clutch, which can reduce drag torque and improve frictional properties between the friction plate and a separator plate of the clutch.
2. Description of the Related Art
FIG. 1 is a cross-sectional side view showing abasic structure of a wet type multiplate clutch 10. In FIG. 1, the wet type multiplate clutch 10 includes a clutch case 20, a piston 30, a center shaft 40, a separator plate 50 and a friction plate 60. A spline groove 21 is formed on the clutch case 20 and is spline-engaged with the separator plate 50. Also, spline teeth 22 are formed on the clutch case 20 and are spline-engaged with the center shaft 40. The separator plate 50 and the friction plate 60 are arranged alternately so that they can be engaged or disengaged with each other, and the friction plate 60 is spline-engaged with a hub (not shown). When the clutch is engaged, hydraulic fluid enters an operating chamber 25 via an oil hole 41 of the center shaft 40 and an oil hole 23 of the clutch case 20 to thereby press the piston 30 to the right direction in FIG. 1. A piston head 31 of the piston 30 presses the separator plate 50 and friction plate 60 towards a stopper ring 24 to thereby engage the clutch. Line X-X indicates a center axis of the center shaft 40.
When the clutch is disengaged by releasing the hydraulic fluid from the operating chamber 25, the piston 30 returns to the left direction in FIG. 1 due to the return force of a return spring 34. The return spring 34 is supported by spring support 35 and a stopper ring 36 is attached to the clutch case 30 as a stopper for the spring support 35. O-rings 32, 33 are provided between the clutch case 20 and the piston 30, respectively. Further, seal members 42, 43 are provided between the clutch case 20 and the center shaft 40.
FIG. 10 shows the first conventional example of the friction plate used for the wet type multiplate clutch. In the friction plate 60, a friction member 62 is attached to a surface of a core plate 61. It is required for the friction plate 60 to reduce the drag torque and to improve cooling capability. Accordingly, an oil passage 63 and an oil groove 64 are formed on the friction plate so that they are arranged alternately as shown in FIG. 10. Note that the oil passage 63 extends outwardly in a direction from an inner peripheral edge 73 to an outer peripheral edge 74 of the friction member 62 with a constant width. The oil groove 64 opens to the inner peripheral edge 73 and closes at a middle of a surface of the friction member 62, and a width thereof is constant. Spline teeth 75 are formed on an inner puerperal edge of the core plate 61.
In the second conventional example as shown in FIG. 11, the oil groove 64 is formed so that its width is larger than that of the oil groove 64 of the first conventional example. Further, U.S. Pat. No. 5,094,331, Japanese Utility Model Unexamined Publications JP-UM-A-1-146019 and JP-UM-A-50-30145 show that the width of the oil passage is formed so as to be decreased towards a direction of an outer peripheral edge to an outer peripheral edge.
It has been carried out to reduce the drag torque by forming oil passages or oil grooves, however, it is required further reduction of the drag torque. In addition, in the friction member having oil passages on its surface, it could reduce the drag torque, however, it tends to have high friction coefficient at an initial stage of a frictional engaging, accordingly, there occurs a problem of biting between the friction plate and the separator plate. Also, this biting depends on engaging pressure at the frictional engaging.